The field effect transistor is typically an element having a semiconductor layer (semiconductor film), a source electrode, a drain electrode, and a gate electrode for each of these electrodes provided with an insulator layer being interposed therebetween, and the like on a substrate. The field effect transistor is used as a logic circuit element in integrated circuits, and widely used as a switching element and the like. The semiconductor layer is typically formed of a semiconductor material. The field effect transistor at present is formed using an inorganic semiconductor material, which is mainly silicon. A thin film transistor having a semiconductor layer created on a substrate such as glass using amorphous silicon in particular is used for displays or the like. In use of such an inorganic semiconductor material, the workpiece needs to be treated at a high temperature or in vacuum during production of the field effect transistor. Therefore, large investment in facility and a large amount of energy during production are necessary, leading to very high production cost. Moreover, because these components are exposed to a high temperature during production of the field effect transistor, a material having insufficient heat resistance such as films and plastics is difficult to use as the substrate. A flexible material that can be bent, for example, is difficult to use as the substrate. Thus, the application area of the field effect transistor is limited.
Meanwhile, field effect transistors using an organic semiconductor material have been studied and developed actively. Use of the organic material can eliminate the treatment at a high temperature, and allow the process at a low temperature, leading to variety of substrate materials that can be used.
As a result, recently, a field effect transistor more flexible, lighter, and more difficult to break than the conventional field effect transistor has been able to be created. In a step of creating the field effect transistor, a method such as application of a solution prepared by dissolving a semiconductor material and printing of the solution by inkjet may be used, and can produce a field effect transistor having a large area at low cost. A variety of compounds for the organic semiconductor material can be selected, and utilization of the properties of the compound and development of the functions that do not exist before are expected.
As an example in which an organic compound is used as the semiconductor material, a variety of organic compounds have been studied. For example, organic materials using pentacene, thiophene, or an oligomer or polymer thereof are already known as a material having hole transport properties (see Patent Literature 1 and Patent Literature 2). Pentacene is an acene aromatic hydrocarbon including 5 benzene rings linearly condensed. It is reported that the field effect transistor using pentacene as the semiconductor material exhibits mobility of charges (carrier mobility) equal to that of amorphous silicon that is used in practice. The field effect transistor using pentacene, however, deteriorates due to an environment, and has problems with stability. The field effect transistor using a thiophene compound also has the same problems, and it is hard to say that these materials have high practicality. Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) being stable in the air and having high carrier mobility has been developed recently, and received attention (see Patent Literature 3 and Non Patent Literature 1). Unfortunately, even these compounds need to have higher carrier mobility for use in applications of displays such as organic ELs. Development of a high quality and high performance organic semiconductor material is demanded from the viewpoint of durability.
Citation list on a DNTT derivative having a substituent includes Patent Literatures 3, 4, and 5. Specific examples of the substituent include a methyl group, a hexyl group, an alkoxyl group, and a substituted ethynyl group. The substituents in the DNTT derivative described as Examples are only a methyl group and a substituted ethynyl group. These groups exhibit only semiconductor properties equal to or less than those of DNTT having no substituent.
Later, Patent Literature 6 describes dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene in Formula 1 (Alkyl DNTT, wherein Alkyl represents a C5 to C16 alkyl group) wherein dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene has properties more excellent than those of the conventional organic semiconductor material in the respect above. Patent Literature 6 shows that the field effect transistor element using this compound is not influenced by the states of the substrate and insulation film during creation of the element (or irrespective of whether a substrate is treated or not), and has extremely improved semiconductor properties; and that the effect is remarkably enhanced by performing a heat treatment during creation of the element.

As described above, these DNTT derivatives useful for the organic semiconductors have been developed, but the conventional production methods have limitation in a method for constructing particularly a thienothiophene structure portion. Namely, a DNTT having a substituent selectively in a position other than the 2,9-positions is difficult to produce, leading to delay of development of a derivative of DNTT. The following three methods of producing a DNTT derivative are mainly known. These will be described below.
The first method is a method in which a derivative is constructed using a starting material tetrabromothienothiophene having a thienothiophene structure from the beginning (Patent Literature 5). In this production method, unsubstituted benzaldehyde causes no problem, but the method has a disadvantage in which use of benzaldehyde having a substituent produces a mixture of DNTT derivatives having substituents at various positions.

The second method is a method of producing a derivative from an ethylene derivative. Most of DNTT derivatives have been synthesized by this method (Non Patent Literature 1, Patent Literature 3, Patent Literature 6, Patent Literature 7, and Patent Literature 8).
For example, Patent Literature 6 discloses that according to the known methods disclosed in Patent Literature 3 and Non Patent Literature 1,2-alkyl-7-methylthio-6-naphthoaldehyde (B) is obtained from 2-alkyl-6-naphthoaldehyde (A), and condensed to obtain 1,2-bis(2-alkyl-7-methylthio-6-naphthyl)ethylene (C). Patent Literature 6 also discloses that 1,2-bis(2-alkyl-7-methylthio-6-naphthyl)ethylene (C) can be further ring closed to obtain a target compound 2,9-dialkyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (2,9-dialkyl DNTT).
Namely, in Patent Literature 6, the compound (B) is obtained by reacting dimethyl sulfide with the compound (A), and the condensate (C) is obtained by McMurry coupling. Further, the target DNTT derivative is obtained by making a ring close reaction in chloroform using the condensate (C) and iodine. Unlike the above first method, the second method is a production method that can produce only a DNTT derivative having a substituent in the target position.

The disadvantage of this synthetic route is that the selectivity of the SMethylation reaction of the compound (A) is approximately 60%, namely, SMethylation in naphthalene at the 7-position as desired occurs only approximately 60%. In approximately 30% of the compound (A), SMethylation undesirably occurs at the 5-position, and approximately 10% of the raw material is recovered. As a result, the compound (B) is extremely difficult to separate and refine.
From above, this method has disadvantages in that the Alkyl-substituted compound (B) cannot be separated by recrystallization that is a method at industrially low cost or the like, needs to be subjected to column refining using an adsorbent (such as silica gel) accompanied by large investment in facility, and cannot be produced at low cost. When the substituent is an aryl group, the separation and the production are more difficult. Additionally, the reaction shown in the reaction formula (2) cannot produce the DNTT having a substituent at 3,10-positions because of limitation in the raw material. This method has such problems, but the method using the compound (B) as the raw material have to be selected in the related art to generate the compound (C) efficiently.
As above, the compound (C) is important in development of the DNTT derivative, but difficulties in synthesis and separation of the compound (B) as the raw material for the DNTT derivative at industrially low cost are known. These difficulties lead to delay of development of the DNTT having a substituent. For this reason, it is easily presumed that if development of an intermediate compound for producing the compound (C) progresses, following this, development of a derivative of the DNTT having a substituent significantly progresses. Development of such a method for producing an intermediate has been required.
The third method is a typical synthesis method using an acetylene derivative (E) (Patent Literature 7). In this synthetic method, it cannot be said yet that an industrial method for producing a Br body (D) as a raw material is established, and a problem of the method is the difficulties in synthesis of an acetylene derivative (E) (Patent Literature 7, Patent Literature 9). Another problem of the method is that cyclization reaction of an acetylene derivative with iodine usually has a low yield (in Patent Literature 7, a yield of approximately 10% to 40%).
